It is the aim of this investigation to characterize nucleotide sequences in ribosomal RNA (rRNA) both in terms of spatial and functional relationships. Our experiments have shown that the rRNA sequences involved in heterologous rRNA-DNA hybridization share certain sequences in common in all eukaryotic species studied thus far. We wish to localize these conserved rRNA and correlate them with functional roles of rRNA. Sequence analysis of conserved regions within prokaryotic 16S rRNA is underway, using E. coli 16S rRNA as a point of reference since most of its nucleic acid sequence is known. Conserved sequences of eukaryotic rRNA will be localized by electron microscopic heteroduplex mapping, using purified Xenopus laevis rDNA.